An embodiment of the present invention relates to a stabilized power supply and in particular for X-ray tubes. An embodiment of the invention can be applied but not exclusively in the field of medical imaging.
An X-ray tube generally comprises a cathode and an anode. The cathode has a filament through which there flows a current known as a heating current. An electrical signal at a high dc voltage of about 150 kV is applied between the anode and the cathode of the tube. The voltage of the signal gives rise to a stream of electrons flowing from the cathode to the anode. In general, an operating point of the tube is defined by the voltage applied between the anode and the cathode and the heating current crossing the filament. Thus, for a given voltage applied to its terminals and an intensity of current traveling through it set by the heating current, the X-ray tube may be considered to be a variable resistor.
To apply an electrical signal of high dc voltage between the anode and the cathode, a stabilized power supply is generally used. This stabilized power supply comprises especially two diode bridges, one inverter and one transformer. To chop a dc voltage electrical signal, the inverter comprises switches, each formed by one diode and one controlled element such as a transistor or a thyristor. Working in sets of two, these switches successively provide for the application of a dc voltage in one direction and then in another, at the terminals of an oscillating circuit in order to create positive and negative half-waves of the current therein. In one example of an embodiment, an electrical signal controls a transistor of the switch. When this signal turns the transistor on, the switch is closed. When the signal turns the transistor off, the switch is opened. When a switch is opened, an over-voltage associated with a variation of a current can be observed at the terminals of this switch. This phenomenon occurs at the instant, called the triggering instant when these diodes go off. During this triggering instant, an overlapping between the voltage at the terminals of this switch and the current crossing this switch causes unnecessary consumption of energy. This unnecessary energy dissipated by the switches is almost negligible for a low change-over switching frequency. However, this energy is not negligible for a high change-over switching frequency, as is the case with the inverter of the X-ray tube.
To attenuate the triggering over-voltage, there is a known resonance inverter. This inverter has a change-over switching frequency that is lower than a resonance frequency of a load. This resonance inverter works well but not as efficiently as desired. This is because the switches of this inverter are connected to a circuit comprising a coil that generates an over-voltage observable at the terminals of the switches. A damping system is furthermore necessary to protect switches during an overlapping of the diodes. This system can give rise to distortions in the electrical signals of the inverter and furthermore introduce energy losses that reduce the efficiency of this inverter.